1. Field of the Invention
The present invention relates to semiconductor modules such as multi-chip modules, for example.
2. Description of the Related Art
DC-to-DC converters are the devices that convert a DC current at one voltage into a DC current with a different voltage level. As DC-DC converters are of high efficiency and are designable to have small sizes, these are adaptable for use as power supply units to be built in various types of small-size electronic equipment (for example, information communication equipment such as notebook personal computers or “PCs” and handheld wireless telephone handsets, also known as mobile phones).
Information communication equipment such as PCs or the like is such that the power supply unit (driving power supply) is becoming lower in voltage and larger in current with advances in miniaturization and high-speed performance of central processing units (CPUs). Although in 1995 the CPU-use power supply voltage remains at the level of 5.0 volts (V) or 3.3V in most applications, the supply voltage is recently lowered to 1.5V due to the appearance of a high-performance CPU with its clock frequency of more than 1 gigahertz (GHz). This trend also requires the current to stay in a class of 100 amperes (A). In addition, in order to offer the supportability of CPU operating speeds, the operation frequency of a power supply circuit also is required to be set at 1 megahertz (MHz) or greater. Due to this, it also becomes important to speed up the switching operations of transistors making up the power supply circuit.
The quest for lower voltages and larger currents of power supply units results in a change in the form of prior known power supply systems. A form having a single power supply unit for supplying electrical power to a plurality of circuits is such that a voltage drop-down can occur even due to the presence of tiny parasitic impedance of wiring lines. This voltage drop might lead to a failure in supplying a required voltage to circuitry, which in turn causes operation errors or malfunctions. To avoid this risk, the decentralization of power supply has advanced at present, for making the transition to a form which employs power supply units corresponding to respective loads and dispose them adjacent to the loads.
Explaining this context while taking a notebook PC as an example, the notebook PC is typically designed to include some major loads such as a CPU, a liquid crystal display (LCD) panel, a hard disk module and others. Separate power supply units (that is, DC-DC converters) corresponding to respective loads are disposed near the loads.
Currently available DC-DC converters for large power use include a noninsulated voltage-drop or “step-down” converter of the type using synchronous rectification schemes. This converter is generally made up of a power metal oxide semiconductor field effect transistor (power MOSFET) of the N-channel type, a Schottky barrier diode (SBD), and an integrated circuit (IC) for pulse width modulation (PWM) control. The individual one of these components is packaged in a way independent of the others. These packaged parts are attached to and mounted on a printed wiring board.
In most cases, a power device such as the power MOSFET used in power supply circuitry experiences heatup, which is caused by power losses due to turn-on resistance and switching operations. To avoid this, it is required that a heat removing device, called the heat sink, is attached to a chip of the power MOSFET for forcing the heat to escape toward the outside of the chip. Note here that a high-frequency multi-chip module is disclosed in FIG. 1 of Published Japanese Patent Application JP-A-11-45976, which module is structured so that a heat sink is attached to a semiconductor chip.